Aspects of the taphonomy of jawless vertebrates

Consideration of taphonomy enhances our understanding of jawless vertebrate history. The hagfish Myxine glutinosa was decayed in a variety of conditions. In all cases, anatomical structures generally regarded as highly decay-resistant degraded before others presumed less so. Even observed decay-resistance, moreover, may not be a reliable criterion for the identification of fossil features. When M. glutinosa carcasses experience taphonomic events that promote exceptional preservation, however, their appearance provides links between features of fossils and anatomical parts of the living animal. W-shaped muscle blocks, for example, can become irregular, Z-, or V-shaped during decay, an observation relevant to interpretations of conodonts with Pikaia gracilens.;On the basis of the response of M. glutinosa to conditions of exceptional preservation, hagfish fossilization is predicted to be rare and biased towards young individuals; this prediction conforms to their observed fossil record. The taphonomy of M. glutinosa carcasses depends heavily upon anatomical factors particular to hagfishes, so these results cannot be readily extended to explain bias in the histories of other vertebrates.;An approach emphasizing taphonomy and incorporating three-dimensional modelling allow features of the problematic fossil taxon Jamoytius kerwoodi to be identified more rigorously. Jamoytius is redescribed as a jawless vertebrate with W-shaped phosphatic scales, ten or more pairs of branchial openings, optic capsules, a subterminal mouth, a terminal nasohypophysial opening, and paired ventrolateral appendages. Cladistic analyses, with the characters of Jamoytius coded as proposed in this study, place it as a sister-taxon to the anaspids.;The orientations at which the feeding apparatuses of the conodont Promissum pulchrum collapsed relative to the sea floor were determined by comparing specimens to a three-dimensional model of the apparatus. Measurements of these collapse orientations provide no evidence that the Soom Shale had a soupy substrate when Promissum was deposited.